This invention relates to the field of cathode ray tube device control circuits and, more particularly, to the elimination of unwanted sync pulses in the circuitry of such devices.
In television and display monitor devices, many methods have been devised for the separation of composite horizontal and vertical sync pulses. In the particular monitor for which the present invention was devised, a computer provides a composite signal containing video and synchronization pulses, very similar to those common to television systems. The video signal provides information which determines the instantaneous brightness of the phosphors on a cathode ray tube as a function of the level of the signal above a given "black" level, while horizontal and vertical synchronization signals are below the black level and appear as positive or negative pulses. The horizontal and vertical pulses are utilized in forming the raster or framework of the video display, and thus must be separated from the composite signal and from each other in order to be utilized in the raster circuitry. A problem arises in separating out the horizontal synchronization signal since the composite signal contains unneeded pulses for this application, and if such pulses are gated through,the raster will not be properly formed. In some composite signals used, the video display is interlaced; i.e., one frame is formed of two fields. The first field is displayed by "painting" one horizontal line on the phosphor face of the CRT by means of an electron beam. The beam is then "darkened" and pulled back to the first edge of the display to be brightened again and create the second horizontal line of the display. This process will be repeated until the field is complete. In the middle of the bottom line of the first field, the beam is again darkened and brought up to the middle of the top line to begin the second field. The horizontal lines of the second field will therefore be displaced from those of the first field by one-half line space, giving greater definition to the visual display. The composite synchronization signal consists of a series of horizontal sync pulses, followed by a vertical sync pulse. Since the horizontal pulse rate must be maintained during the vertical pulse, the horizontal pulses are inverted during that period, foiling serrations in the vertical pulse which allow the horizontal sync rate to remain locked. Extra horizontal synchronization pulses are required which would not be needed if the display fields were not to be interlaced. In this application, the raster is not to be formed of interlaced lines, thus, the unneeded horizontal pulses during the vertical pulse must be gated out while allowing all desired pulses to pass through.
An additional problem arises in that removal of the undesired pulses must be accomplished in such a way that the horizontal phase/frequency can be adjusted, as by the device user, without affecting the process of eliminating the unwanted horizontal sync pulses.